Nedarim 59 - Vows are bad
In trying to answer the question about an onion - whether new growth in it removes the prohibitions of the old one - the Talmud compares it to tithes, to Kohen's portion (terumah) and finally to our original ruling about vows that one takes against a fruit . In this last case, all that grows from a fruit was forbidden, which must prove that new growth does not nullify the old one.
However, even this proof does not stand. Vows are different, because they are inherently bad, and no regular laws can be derived from them. Why are vows bad? - Just like Rabbi Nathan said - one who makes a vow is like one who builds a private altar for sacrifices (which is now forbidden, there should be only one Altar in Jerusalem), and one who fulfilled his vow is as if he brought a sacrifice on this altar.
To explain, one who builds a private altar probably thinks that he does a mitzvah by worshiping in this manner. And one who makes a vow also thinks that he did a great thing by creating another prohibition for himself. In truth, the argument should be just the opposite: there are enough prohibitions already, and one should not create more.
Art: Still Life: Fruit by Gustave Courbet